Secondary batteries are used as a power supplier of portable electronic devices for information communications such as PDA, cellular phones, notebook computers, etc., electric bicycles, electric automobiles, and the like. Therefore, there is a growing demand for the secondary batteries capable of repeatedly charging and discharging. In particular, since the performance of the devices depends on secondary batteries, high performance secondary batteries are required. The characteristics required for secondary batteries include charge-discharge characteristics, life characteristics, high-rate characteristics, thermal stability at high temperature and the like. In addition, lithium secondary battery has been drawn attention in terms of high voltage and energy density.
Lithium secondary batteries are classified into lithium batteries using lithium metal as a negative electrode and lithium ion batteries using carbon capable of intercalating/deintercalating lithium ion as an interlayer compound. Further, lithium secondary batteries are often classified into liquid type batteries, gel type polymer and solid type polymer batteries according to the used electrolytes.
Lithium secondary batteries are commonly comprised of a positive electrode, a negative electrode, an electrolyte, a separator, a packaging material, etc. The positive electrode is prepared by dispersing a mixture of a lithium transition metal composite oxide such as LiCoO2, LiMn2O4, LiNiO2, or LiMnO2 as a positive electrode active material, an electrically conductive agent and a binder into a current collector. The active materials have a high electrochemical potential during intercalation/deintercalation reaction by lithium ion. The active material for a negative electrode includes lithium, carbon, or the like, which has a low electrochemical potential.
The electrolyte is prepared by dissolving a lithium ion-containing salt such as LiPF6, LiBF4, LiClO4, etc., in a polar organic solvent such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methylethyl carbonate, etc. The separator mainly uses polyolefin-based polymer such as porous polyethylene for ion conducting layer and electrical insulation between anode and cathode. The packaging material which protects the contents of battery and provides a path of electrical signal includes a metal can made of iron or aluminum, or a multilayer of aluminum foil and polymer film.
However, lithium secondary batteries have many problems, e.g., the life of battery may be shortened by a repeated charge/discharge cycle, particularly at high temperature. This is because electrolyte is decomposed, active material is deteriorated, or internal resistance of battery increases, due to moisture present in the battery, etc.
Many efforts to solve these problems have been made. For example, Korean Patent Publication No. 10-277796 discloses a method for coating metal oxides such as Mg, Al, Co, K, Na, Ca, etc., onto the surface of positive electrode active material by heat treatment. A technique for increasing energy density and high-rate characteristics by adding TiO2 to LiCoO2 as an active material is disclosed in the “Electrochemical and Solid-State Letters, 4(6), A65-A67 (2001). A technique for improving battery life by surface-treating natural graphite with aluminum is disclosed in the Electrochemical and Solid-State Letters, 4(8), A109-A112 (2001). However, there still exist problems such as life deterioration and the generation of gas due to electrolyte decomposition during charging and discharging cycle.
In addition, a problem that electrolyte is oxidized due to the reduction in battery capacity during charging, and the resulting acid dissolves active material is disclosed in the Journal of Electrochemical Society, 143(1996), p 2204.